Electric motor protector sensor

ABSTRACT

An electric motor protector sensor for use in pressurized atmospheres comprises a small thermistor of low-thermal inertia which is carried in an opening in a thin, inner layer of flexible, temperature-resistant, electrically insulating material to expose portions of the thermistor on opposite sides of the thin insulating layer. Ends of lead wires are connected to these exposed thermistor portions, the lead wires extending away from the thermistor along opposite sides of the thin insulating layer to be connected at their opposite ends to the conductive cores of insulated conductors. Thin, flexible, disc-shaped foils of copper of much larger diameter or surface area than the thermistor are disposed on each side of the thin insulating layer in heattransfer engagement with respective exposed portions of the thermistor and with portions of the lead wires, thereby to provide heat-collecting members of substantial surface area in the sensor. Outer thin layers of similar insulating material sandwich the thin insulating layer and are bonded to opposite sides of the inner insulating layer around the copper foils, thermistor and leads for sealing the sensor. Heat-shrunk tubes of insulating material are disposed in sealing relation around portions of the insulated conductors and portions of the outer insulating layers of the sensor to complete sealing of the sensor.

United States Patent tubes of insulating material are disposed insealing relation around portions of the insulated conductors andportions of the outer insulating layers of the sensor to completesealing of the sensor.

WaseleskLJr. et al. 1 Feb. 29, 1972 '[541 ELECTRIC MOTOR PROTECTOR [57]ABSTRACT SENSOR An electric motor protector sensor for use inpressurized at- [72] Inventors: Joseph W. Waseleski, Jr., Man field,mospheres comprises a small thermistor of low-thermal inertia Mass;Ralph E. Charnley, Edmond, R.I. which is carried in an opening in athin, inner layer of flexible,

' temperature-resistant, electrically insulating material to ex- [73]Asslgnee 3 limrunienw incorporated Dallas pose portions of thethermistor on opposite sides of the thin insulating layer. Ends of leadwires are connected to these ex- [22] Filed: Jan. 23, 1970 posedthermistor portions, the lead wires extending away from the thermistoralong opposite sides of the thin insulating layer [21] Appl' 5428 to beconnected at their opposite ends to the conductive cores of insulatedconductors. Thin, flexible, disc-shaped foils of [52] 0.8. CI. ..338/25,338/26, 3l7/4l copper of much larger diameter or surface area than the fCl thermistor are disposed on each side of the thin insulating [58]Field of Search ..3l7/4l', 338/22, 23, 25, 26 layer in heat-transferengagement with respective exposed portions of the thermistor and withportions of the lead wires,

[56] Reiel'mces Clted thereby to provide heat-collecting members ofsubstantial surface area in the sensor. Outer thin layers of similarinsulating UNiTED STATES PATENTS material sandwich the thin insulatinglayer and are bonded to 3,521,212 7/1970 Waseleski, Jr. et al. ..3l7/4lX opposite sides of the inner insulating layer around the copper3,430,336 3/ i969 Riddel ..338/22 X foils, thermistor and leads forsealing the sensor. Heat-shrunk T-l-ll- I -1- I -1- r l A I n PATENTEDFEB 2 9 I972 sum 1 OF 3 PAIENTEDraazs ma SHEET 2 BF 3 ELECTRIC MOTORPROTECTOR SENSOR In a known motor protection sensor in which athermistor is disposed in an opening in a thin inner layer of insulatingmaterial to expose portions of the thermistor on opposite sides of theinsulating layer, outer thin layers of similar insulating material areprovided with copper foils which are deposited thereon. These outerinsulating layers are arranged to sandwich the inner insulating layers,the deposited copper foils carried by the outer insulating layers beingsoldered to respective exposed portions of the thermistor on oppositesides of the inner insulating layer. The outer layers of the sensor arealso bonded to the inner insulating layer around the copper foils forsealing the sensor. Stern portions of these deposited copper foilsextend to edges of the insulating layers where they are electricallyconnected to appropriate conductors. In this arrangement, the copperfoils not only serve as heat-collecting members but also serve aselectrical leads extending from the thennistor. When these known sensorsare located within motor windings, the heat-collecting copper filsreadily conform to the winding configuration and cooperate with lowthermal inertia of the thermistor to permit very rapid response of thesensor to rising motor temperatures. This prior art sensor thereforesignificantly reduces overshoot of motor heating when the sensorindicates that motor temperature has risen to the point where motoroperation should be interrupted. However, these known sensors arefrequently used in pressurized freon atmospheres and the like, inrefrigeration motors for example, and it is found that, despite thesealed nature of the sensor, some pressurized freon tends to enter thesensor between the outer layers of the sensor-insulating material. If atemporary loss of gas condition thereafter occurs in the sensorenvironment, it is found that rapid expansion of the freon gas trappedwithin the sensor tends to separate the outer insulating layers of thesensor with resulting fracture of the electrical connection between thethermistor and the copper foils deposited on the insulating layers. Uponrestoration of the desired pressurized atmosphere around the sensor, itis found that electrical connections between the copper foils and thethermistor are frequently not restored and the sensor becomesinoperative. Where the sensors are positioned within the motor windingduring assembly of the motor, this failure of the sensor results in lossof the entire motor winding.

It is an object of this invention to provide a novel and improvedelecuic motor protection sensor; to provide such a sensor which ischaracterized by very rapid response to rising motor windingtemperatures; to provide such a sensor which is useful in pressurizedatmosphere without risk of electrical failure upon the occurrence ofloss of pressure in the sensor environment; and to provide such a sensorwhich is of simple, reliable, rugged and inexpensive construction.

Other objects, advantages and details of the motor protection sensor ofthis invention appear in the following detailed description of preferredembodiments of the invention, the description referring to the drawingsin which:

FIG. 1 is a plane view of the sensor of this invention;

FIG. 2 is a section view along line 2--2 of FIG. 1;

FIG. 3 is a section view along line 3-3 of FIG. 1;

FIG. 4 is a section view along line 4-4 of FIG. 1;

FIG. 5 is a diagrammatic view illustrating use of the sensor within amotor winding; and

FIGS. 6 and 7 are graphs illustrating operating characteristics of thesensor.

Referring to the drawings, 10 in FIGS. 1-5 indicates the novel andimproved electric motor protection sensor of this invention which isshown to include a thermistor 12 which preferably comprises a smallpellet of semiconductive material having a selected positive or negativetemperature coefficient of resistance. Preferably, the thermistor isadapted to display gradually changing electrical resistance in responseto change of thermistor temperature within a selected range and is thenadapted to display a much more rapid change in electrical resistance inresponse to further change of temperature. Preferably, for example, thethermistor embodies a lanthanide-doped barium titanate, barium strontiumtitanate, bariurn lead titanate or the like. The size of the thermistoris preferably very small so that the thermistor displays low thermalinertia but the size of the thermistor in the sensor can vary within thescope of this invention in accordance with the electrical parameters ofthe motor control system in which the sensor is to be used. A practicalthermistor useful in sensors finding wide commercial applicationcomprises a pellet of lanthanide-doped barium titanate 0.050 inches inlength and 0.070 inches in diameter. However, other thermistors of thesame material ranging in length from 0.040 to 0.060 inches and rangingin diameter from 0.050 to 0.080 inches are-also useful in manyapplications.

In accordance with this invention, the thermistor I2 is held in a smallopening 14 in a thin layer 16 of electrical insulating material. Thismaterial is capable of withstanding substantially elevated temperaturesto which the material might be exposed in an overheating electric motor.For example, a preferred insulating material for this purpose comprisesa polyimide resin such as is sold commercially under the trade nameKapton." Other materials useful in the insulating layer 16 includepolyethylene tetraphthalate such as is sold under the trade name Mylaror a fluorinated aliphatic hydrocarbon such as tetrafluoroethylene soldunder the trademark Teflon. In a practical embodiment of the sensor ofthis invention, the thin inner layer 16 of the sensor preferably has athickness in the range from approximately 2-6 mils. The inner insulatinglayer 16 of the sensor is preferably of generally round configurationhaving a radially extending stern portion as indicated at 18 in FIG. 1.

In accordance with this invention, opposite end portions of thethermistor 12 are exposed at either side of the thin insulating layer 16in the sensor, and thin, elongated leads 20 are electrically connectedto respective exposed portions of the thermistor to extend away from thethermistor along opposite sides of the thin insulating layer 16.Preferably for example, the leads 20 comprise No. 28 Gage copper wiresand are soldered as indicated at 22 by use of any conventional lead-tinor gold-tin solder material. An appropriate solder for example comprisesa gold-tin eutectic material containing percent gold and 20 percent tin.

In accordance with this invention, a pair of copper foils 24 ofrelatively larger diameter than the thermistor 12 are disposed onrespective sides of the thin insulating layer 16 in heat-transferengagement with the thermistor 12 and with portions of the thermistorleads 20. For example, each of the copper foils preferably has athickness in the range from 0.40 to 0.75 mils and has a diameter ofapproximately 0.50 inches.

In accordance with this invention, outer layers 26 of electricallyinsulating material similar to the material embodied in the thin innerinsulating layer 16 are superimposed upon respective opposite sides of athin insulating layer 16 as shown. Preferably, for example, the outerinsulating layers 26 are embodied in a single sheet of material which isfolded along an edge indicated at 28 in FIGS. 1 and 2 so that portionsof the sheet falling upon each side of the thin insulating layer 16constitute the outer insulating layers 26 of the sensor 10. In this way,the outer insulating layers 26 sandwich the inner insulating layer 16,themiistor 12, the leads 20, and the copper foils 24 between the outerinsulating layers. The outer insulating layers 26 are then bonded to theinner insulating layer 16 of the sensor around the leads 20 and thecopper foils 24 as indicated at 29 in FIG. 3 for sealing the inner andouter insulating layers together. An appropriate bonding material usefulfor bonding these insulating materials together comprises aheat-scalable fluorinated aliphatic hydrocarbon such as it is commonlysold under the trademark Teflon.

If desired, in the preferred embodiment of this invention, a pair ofinsulated conductors 30 having conductive cores 32 are disposed onrespective sides of the stern portion of the thin inner insulating layer16 with the conductive cores thereof secured in electrically conductiverelation to the ends of the thermistor leads 20 oppositely of thethermistor 12. For example, the cores of the insulated conductors arepreferably soldered to respective leads 20 within any conventionallead-tin solder or the like as indicated at 34 in FIGS. 1, 2 and 4.

In a preferred embodiment of this invention, a tube or sleeve 36 ofheat-shrinkable, electrically insulating material is fitted aroundportions of the insulated conductors 30 and is fitted over the stemportions of the insulating layers 16 and 26 of the sensor 10, the tube36 then being heated for shrinking the tube into sealing relation to theinsulated conductors 30 and to the insulating layers of the sensor 10. Apreferred material for this sleeve 36 comprises cross-linkedpolyethylene tetraphthalate which is sold under the trademark Mylar. Inassembling the sensor 10, the sleeve 36 is fitted over the stem portionsof the insulating layers 16 and 26. The sleeve is then folded back onitself to permit access to the ends of the leads 20 located at the endsof the insulating layer stem portions. The insulated conductors 30 arethen positioned over the ends of the leads 20 and the conductive coresthereof are soldered to the leads 20 as indicated at 34. The sleeve 36is then unfolded from itself to extend the sleeve over the insulatedportions of the insulated conductors 30. Heating of the insulatingsleeve then serves to seal the sleeve to the insulated conductors and tothe stem portions of the sensor insulating layers. Other sleevematerials which may be cross-linked, expanded and then shrunk by heatingare polyamide resins such as are sometimes sold under the trade nameNylon or the material described above as Teflon.

The sensor above described has a number of advantages as illustrated inFIG. 5. That is, because the sensor is thin and flexible, the sensorinterleaves readily between insulated convolutions 38 of a motor winding40 shown in FIG. 5. Thus the copper foils 24 of the sensor achieve anefficient heatexchange relationship with the windings of various typesof motors and readily conduct heat from the windings, the heatcollectingfunction of the copper foils permitting conduction of a substantial anduniform amount of heat to portions of the thermistor engaged by thefoils. In general, increase of the ratio of the heat collecting surfacesof the copper foils relative to the thermal mass of the thermistor 12 tobe heated is highly desirable. If the heat-collecting area provided inthe sensor were too small, the thermistor would not have enough heatdelivered to it to drive its temperature up as rapidly as might benecessary to closely follow the rapidly rising temperature of the motorwinding. There would thus be a lag between the arrival of the motorwinding at a dangerous temperature and the sensing of that temperatureby the thermistor. This lag could permit the motor winding to overshootits maximum desirable temperature and could result in overheating of themotor winding before motor protective devices operably connected withthe sensor could interrupt motor operation. Equally important, if onlythe small surface area of the thermistor were available to perform aheat-collecting function, the positioning of the thermistor directlybetween convolutions of a motor winding could not be achieved with aconsistent degree of thermal contact with the motor winding. That is,the small thermistor surfaces might be positioned directly adjacentwinding convolutions or might fallin interstices between theconvolutions, whereby the sensing characteristics of the sensor relativeto the winding would vary from motor to motor. In a desirableconstruction, desirable proportions for the sensor include use of athermistor 0.070 inches in diameter and 0.050 inches in length betweenconductive copper foils 24 of approximately 0.500 inches in diameter,the copper foils being about 0.4 mils thick and being sandwiched betweenouter insulating layers 26 of the sensor which are approximately l. l 25inches in diameter. Desirably the outer insulating layers of the sensorhave a thickness of from 2 to 6 mils. These proportions of sensorcomponents provide the highly desirable combination of small thermistormass to achieve low thermal inertia and large heat-collecting surfacesprovided by the copper foils, the foils assuring appropriate averagingof thermal contact within a motor winding to avoid any scatter effectsuch as might result from poor positioning of the small thermistorwithin the winding. Note that a thermistor of smaller size than abovedescribed could be difficult to solder and might provide a choke effecton heat transfer from the motor winding to the thermistor, which chokeeffect might tend to increase the time constant of the thermistor.

For example, as is illustrated in FIG. 6, when the mass of thethennistor is small, the thermistor can display a very rapid rate risein temperature as indicated by curve M in FIG. 6. However, when thethermistor size is small, the thermal contact which can be obtainedbetween the thermistor and the convolutions of a motor winding becomesincreasingly variable so that as illustrated by curves L and N in FIG.6, the same small thermistor displays variable response to increasingmotor winding temperature depending upon the degree of thermal contactbetween the thermistor and the winding. That is, where the thermistor isvery small, the scatter effect encountered in properly positioning thethermistor within a motor winding is substantial and difiiculty isencountered in utilizing the thermistor to follow a particular rate ofmotor winding temperature rise. However, as is illustrated in FIG. 7,the use of copper foils to perform a heat-collecting function for athermistor of low thermal inertia permits stabilizingof the timeconstant of the sensor and reduces variation in time constant whichresults from variations in positioning of the sensor within a motorwinding. That is, while a small thermistor utilized without theheatcollecting copper foils of the sensor of the present invention mightachieve a time constant as indicated at point F on the curve of FIG. 7,the small thermistor would be subject to the large scatter effectillustrated in FIG. 6. In the sensor of the present invention, where theratio of a heat-collecting surface to the thermistor mass issubstantially greater, the time constant of the thermistor tends to besomewhat lower, as indicated at point G in FIG. 7, but is still quitehigh. In addition, the sensor 10 of this invention, having a much higherratio of heat-collecting surface to thermistor mass displays much lessscatter effect when positioned within a motor winding. That is, thethermistor of the present invention achieves a highly desirablecombination of low thermal inertia and high heat-collecting surfacewhich assures that the sensor is readily positioned within a motorwinding to achieve a high and consistent thermistor time constant forpermitting the sensor to follow rapidly increasing motor windingtemperature in a consistent manner.

The sensor of the present invention has the additional advantage that,should the sensor be utilized in a pressurized environment, the sensoris not adversely effected by temporary loss of pressure conditions inthe environment. That is, if pressurized gas from the sensor environmentshould enter between the outer insulating layers of the sensor despitethe sealed nature of the sensor, a temporary loss of gas condition inthe sensor environment might permit the gas trapped between thesensor-insulating layers to expand the layers for moving the copperfoils 24 out of engagement with the thermistor 12. However, theelectrical connection between the thin, elongated leads 20 and thethermistor 12 would be uneffected by such expansion of the insulatingsensor layers. Thus, upon restoration of the pressurized atmosphere inthe sensor environment, the outer insulating layers of the sensor wouldbe restored to their initial position, thereby restoring properheattransfer engagement of the copper foils 24 with the thermistor andwith the leads 20 without requiring any restoration of electricalconnection to the thermistor. In this way, it is found that the sensor10 of this invention is especially adapted for use in pressurizedenvironments.

It should be understood that a preferred embodiment of this inventionhas been described by way of illustration but that all modifications andequivalents thereof which fall within the scope of the appended claimsare included within the scope of this invention. For example, althoughthe leads 20 are shown to be of round configuration, these leads couldbe narrow and flat throughout their length or could be flattened attheir point of connection to the thermistor l2 and conductive cores 32.Further, although the thermistor 12 is shown to be fitted within anopening in the inner insulating layer 16 of thesensor, a convenientmethod of thermistor assembly mounts the thermistor 12 extending throughcentral apertures in two superimposed small discs of insulatingmaterial. The leads 20 are then attached to-portions of the thermistorexposed on opposite sides of the pair of insulating discs. This assemblyis then disposed so that the thermistor extends through an aperture 14in the thin insulating layer 16 with one of the small insulating discsbeing disposed at either side of the thin insulating layer 16 forretaining the thermistor in the aperture 14, the leads 20 extendingalong each side of the layer 16. Other modifications which might assistin assembly of the sensor are also within the scope of this invention ifthey fall within the scope of the appended claims.

I claim:

l. A sensor comprising a thermistor material having selected temperaturecoefficient of resistance properties; an inner layer of flexible,electrically insulating material having an opening containing saidthermistor and exposing portions of said thermistor of selected area onopposite sides of said inner layer; a pair of elongated electricallyconductive lead members which are secured in electrically conductiverelation to said thermistor portions exposed on respective oppositesides said inner layer and which extend from said thermistor portionsalong respective opposite sides of said inner layer of flexibleelectrically insulating material toward an edge of said inner layer ofmaterial; a pair of discrete, flexible foils of heatconducting materialof relatively much greater area than said thermistor portions disposedon respective opposite sides of said inner layer in heat-transferengagement with said thermistor portions and portions of said respectiveleads and in physical engagement with portions of said inner layeraround said inner layer opening; and a pair of outer layers of flexible,electrically-insulating material disposed on respective opposite sidesof said inner layer, said outer layers being bonded to said inner layeraround said foils and leads for sealing said foils and thermistortherebetween.

2. A sensor according to claim 1 wherein the thickness of each foil isin the range from 0.40 to 0.75 mils.

3. A sensor according to claim 2 wherein the thickness of said inner andouter insulating layers is in the range from 2.0 to 6.0 mils.

4. A sensor according to claim 3 having electrical conductors withconductive cores and electrically insulating coatings on the conductivecores arranged so that the conductive cores of the insulated conductorsare secured in electrically conductive relation to respective leadsadjacent an edge of said inner layer.

5. A sensor as set forth in claim 4 having an insulating sleeve shrunkin sealing relation to said insulated conductors and said outer sensorlayers for sealing said sensor to said conductors.

6. A sensor as set forth in claim 5 wherein said thennistor displays aselected temperature coefficient of resistance over a selectedtemperature range and displays a relatively different temperaturecoefficient of resistance over another selected temperature range.

1. A sensor comprising a thermistor material having selected temperaturecoefficient of resistance properties; an inner layer of flexible,electrically insulating material having an opening containing saidthermistor and exposing portions of said thermistor of selected area onopposite sides of said inner layer; a pair of elongated electricallyconductive lead members which are secured in electrically conductiverelation to said thermistor portions exposed on respective oppositesides said inner layer and which extend from said thermistor portionsalong respective opposite sides of said inner layer of flexibleelectrically insulating material toward an edge of said inner layer ofmaterial; a pair of discrete, flexible foils of heatconducting materialof relatively much greater area than said thermistor portions disposedon respective opposite sides of said inner layer in heat-transferengagement with said thermistor portions and portions of said respectiveleads and in physical engagement with portions of said inner layeraround said inner layer opening; and a pair of outer layers of flexible,electrically-insulating material disposed on respective opposite sidesof said inner layer, said outer layers being bonded to said inner layeraround said foils and leads for sealing said foils and thermistortherebetween.
 2. A sensor according to claim 1 wherein the thickness ofeach foil is in the range from 0.40 to 0.75 mils.
 3. A sensor accordingto claim 2 wherein the thickness of said inner and outer insulatinglayers is in the range from 2.0 to 6.0 mils.
 4. A sensor according toclaim 3 having electrical conductors with conductive cores andelectrically insulating coatings on the conductive cores arranged sothat the conductive cores of the insulated conductors are secured inelectrically conductive relation to respective leads adjacent an edge ofsaid inner layer.
 5. A sensor as set forth in claim 4 having aninsulating sleeve shrunk in sealing relation to said insulatedconductors and said outer sensor layers for sealing said sensor to saidconductors.
 6. A sensor as set forth in claim 5 wherein said thermistordisplays a selected temperature coefficient of resistance over aselected temperature range and displays a relatively differenttemperature coefficient of resistance over another selected temperaturerange.